1. Field of the Invention
This invention relates broadly to automatic shear control. More particularly, it relates to a method and apparatus for controlling a dividing shear which makes one or more cuts into a moving workpiece during a corresponding number of shear cutting cycles. The moving workpiece may be a steel bar, as referred to below, or a steel billet, sheet, plate and the like. The term "shear cutting cycle" defines shear movement starting and ending at a home position and having a cutting position therebetween.
2. Description of the Prior Art
Contemporary hot steel bar mills roll a moving workpiece at speeds of from about 1000 to about 4000 feet per minute (305 to 1219 meters per minute). A dividing shear is used to cut a continuously moving bar into one or more lengths according to cutting order data. After each cutting cycle the dividing shear must return to its home position as rapidly as possible and be ready for the next cut. Any instability of the dividing shear control, such as oscillations and/or overshoot at the home position of a cutting cycle, results in inadequate bar length control at the aforesaid speed range. More importantly, such instability of the dividing shear may cause a cobble in the bar mill which has serious economic and operating consequences.
Heretofore, dividing shear control equipment used a velocity signal proportional to the difference between the present position of the dividing shear and its home position as an error signal to drive the dividing shear to its home position after cutting the workpiece. However, a dead band is located near the home position where no signal is sent to the dividing shear control equipment. Consequently, the dividing shear coasted beyond the dead band and generated a negative velocity control signal proportional to difference between shear present position and its home position. The negative velocity control signal caused a reversal in the dividing shear drive back toward the home position.
If the dividing shear coasted in reverse beyond the home position, a cycle was produced and in some instances a self-sustained oscillation was generated about the home position of the dividing shear operating cycle. This self-sustained oscillation, or limit cycle as is known in the control art, is the result of electrical and/or mechanical nonlinear dynamic changes brought about by changes in mill environment. For example, dynamic changes such as changes in dividing shear lag, cutting order data including bar dimensions and/or grade and other mill operating conditions may produce a control limit cycle, particularly dynamic changes in dividing shear lag.